The ball_ud blocks (ball_ud and its three shifted variants). The ball goes straight up and down: rising at two units a frame to a ceiling, then falling under gravity until it lands, which sends it up again with a thud. Bit 2 of +13 is set while it is rising. It flips between its two frames every frame and kills on contact.

The ceiling is shared. $5BBD is cleared on entering a room, and the first ball to run sets it to 32 above its own height; every ball in the room then turns at that same height.